Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
A reliable semiconductor device in which stable electrical characteristics are applied to a transistor using an oxide semiconductor is disclosed (e.g., see Patent Document 1). In the semiconductor device, oxide semiconductor films with different compositions are stacked so that an oxide semiconductor film containing many In is provided on the channel side and an oxide semiconductor film containing many stabilizers such as Ga is provided on the back-channel side.